High intensity focused acoustic waves, such as ultrasonic waves (acoustic waves with a frequency greater than about 20 kilohertz), may be used to therapeutically treat internal tissue regions within a patient. For example, ultrasonic waves may be used to ablate tumors rather than requiring the patient to undergo invasive surgery. For this purpose, piezo-ceramic transducers are driven by electric signals to produce ultrasonic energy and are placed externally to the patient but in close proximity to the tissue to be ablated. The transducer can be geometrically shaped and positioned such that the ultrasonic energy is focused at a “focal zone” corresponding to a target tissue region within the patient. The target tissue region is heated until the tissue is necrosed. The transducer may be sequentially focused and activated at a number of focal zones in close proximity to one another.
This series of sonications is used to cause coagulation necrosis of a tissue structure, such as a tumor, without damaging surrounding tissue. To achieve this, ultrasonic energy must be properly focused and applied to the correct location. Further, the ultrasonic beam must have a suitable sharpness and shape so that unexpected hot spots outside the tumor zone are not generated.
One important factor impacting the effectiveness of the transducer and the therapy provided to the patient is the ability to accurately and reliably shape the transducer beam and focus the output of the transducer at the focal zone corresponding to the target tissue. Transducer elements that are not properly configured or controlled can lead to improper focus location and reduced focus quality, resulting in less effective therapy. Further, they can result in secondary hot spots or concentrated areas of heating beyond the focal zone, which can cause damage to surrounding healthy tissue.
One source of transducer output errors results from transducer elements moving or shifting from their expected location. For example, assuming a transducer has a spherical shape, the software that drives each transducer element is typically configured to activate individual transducer elements based on the elements being positioned according to a spherical model or design. In practice, however, transducer elements often are not positioned according to a spherical model. Instead, the actual location of one or more transducer elements may be shifted from their expected locations during manufacturing, use and repair. Further, the location of transducer elements can shift as a result of the element being deformed by heat. These changes can result in permanent focusing errors that are not compensated by software that is programmed to drive individual elements based on a pre-determined spherical model
Even slight location deviations can have significant effects on the quality of the transducer output and may cause secondary hot spots. For example, FIG. 1 generally illustrates a cross-sectional view of a surface of a spherical transducer 10 having a distortion or deviation 12. Thus, the actual location 14 of the transducer surface is shifted downwardly from the expected location 16 that is based on a pre-determined spherical model. In this example, the distortion is greatest at the middle of the transducer 10 and is one-dimensional or vertical. FIG. 2 illustrates another example of a spherical transducer 20 having another type of distortion 22. This type of distortion 22 is similar to the distortion 12 shown in FIG. 1 except that the distortion 22 is less pronounced at the middle of the transducer. This type of distortion 22 can also cause focus errors, as well as secondary hot spots.
With reference to FIG. 1, for example, a transducer 10 having a deviation “δ” of about 1 mm, a radius of curvature “R” of about 160 mm and an aperture “A” of about 120 mm can cause the location of the ultrasonic output to be shifted by about 13 mm. In other words, a relatively small deviation can alter the output of a transducer in such a manner that the resulting change in the output is substantially larger than the deviation itself.
Additionally, even if the physical structure of the transducer surface is as expected, individual transducer elements can be wired improperly, e.g., at the time of manufacture or during service. Improper wiring can result in transducer elements being driven by signals intended for other transducer elements, thereby causing location and focus errors.
One attempt to solve these problems involves focusing the transducer in water at a focal point and using a hydrophone to locate the focal point of maximum intensity. Each transducer element is then separately activated at the maximum intensity point, and the phase of each signal is measured. The measured phase for each element is compared to the expected phase, and the driving signal is adjusted to compensate for the phase deviation. This “phase measurement and phase adjustment” approach, however, has a number of shortcomings.
One shortcoming is that this technique applies to focus locations that are close to the location of the focal point during calibration. However, phased array transducers allow electronic steering of the focal point, and this technique may not be effective when the location of the focal point is steered to different locations beyond the point used during calibration. Moreover, this technique requires an accurate scanner and electronics in order to locate the focal point. Consequently, the “phase measurement and phase compensation” approach can be time consuming and may not be practical during actual use in the field.
Thus, while known transducer arrays have been successfully used in the past, they can be improved, particularly the manner in which variations in transducer element location caused by movement and deformations are compensated to maintain the quality of the transducer output while reducing or eliminating secondary hot spots.